Glycerol kinetics were evaluated during high intensity exercise in five untrained and five endurance-trained subjects. Glycerol rate of appearance (Ra) in plasma was determined by infusing (2H~]glycerol during rest and 60 min of cycle ergometer exercise performed at 70 % VO2 peak Mean plasma glycerol concentration was greater in trained than untrained subjects throughout exercise (P < 0.05). Average glycerol Ra during exercise and the integrated lipolytic response to exercise, expressed as total glycerol Ra above baseline, were both greater in trained (7.85+0.72 Umol-kg-1.min-1 and 289+50 umol.kg-1 h-1 , respectively) than in untrained (5.68+0.90 umol.kg-1.min-1, and 198+31 umol.kg-1. h-1' respectively) subjects (P < 0.05). We conclude that whole-body lipolytic rates are greater in endurance-trained athletes than in sedentary controls during high intensity exercise performed at the same relative intensity.